Operating companies in telephone communications systems typically receive voice signals on a plurality of lines, convert those voice signals to digital data and transmit the digital data in a time multiplexed fashion over a digital communications line. At the remote end, a like terminal receives the digital data, converts the data to pulse amplitude modulated (PAM) form and distributes the PAM signals to channel units. Each channel unit corresponds to a specific time slot of the time multiplexed digital signal and a specific voice signal line and converts a PAM signal to voice.
A widely used terminal is the modular D4 terminal. In that terminal, a channel unit module is provided for each voice signal line to provide the conversion between the PAM signals and the voice signals. Other channel units within the terminal may be devoted to lines which only transmit digital data and thus do not handle voice signals. The terminal also includes receive and transmit modules which determine time slots and frames of time slots of the multiplexed digital signal. The receive units convert the digital signals to PAM signals and distribute those PAM signals to the channel units corresponding to the time slots. The transmit units convert the PAM signals to pulse code modulated (PCM) signals and multiplex those signals into their corresponding time slots.
Prior to installation, switches of the channel units of D4 terminals are set to provision the units for proper gain adjustment, equalization and other operating parameters. Cumbersome tests are then performed, and if further adjustment is required, the units must be removed for resetting of the switches. More recently, replacement terminals allow for automatic testing and setting of the channel unit switches, but replacement terminals require disruption of the existing plant and disposal of nonconforming yet operable components.